The present invention relates to a device for handling substrates, in particular semiconductor slices, in an apparatus used for the chemical vapour deposition (CVD) of semiconductor material onto the said substrates and a method for operation of the said device. It especially relates to a device for handling substrates used in an epitaxial reactor and, in particular, relates to an epitaxial reactor for performing the chemical vapour deposition (CVD) of materials onto the said substrates, preferably silicon substrates used in the manufacture of semiconductor components, such as chips for integrated circuits.
More particularly, the present invention relates to a device used in epitaxial reactors such as those covered by the International Patent Application WO 96/10659 filed on Sep. 14, 1995, with the title xe2x80x9cEpitaxial reactor provided with flat disk-shaped susceptor and having a gas flow parallel to the substratesxe2x80x9d. With the aid of the present device, the abovementioned epitaxial reactor becomes a reactor of the xe2x80x9ccassette-to-cassettexe2x80x9d type because the cassettes containing the as yet unprocessed substrates are positioned inside the reactor and, during a product loading cycle, a first mechanized arm, or robot, not forming part of the present invention, is used to transport the substrates from a storage rack or xe2x80x9ccassettexe2x80x9d to a purging chamber and a second mechanized arm, or robot, carrying externally a gripping and transportation means, forming the subject of the present invention, for transporting the substrates from the purging chamber to the susceptor, whereas, during an unloading cycle, the second robot transports the substrates from the susceptor to the purging chamber and subsequently the first robot transports the said substrates, which have undergone processing, from the purging chamber to one of the cassettes, all of which occurring without manual intervention of an operator supervising operation of the reactor.
The invention may be applied in particular to cold-wall CVD systems, preferably, to reactors which are able to provide epitaxial growth on substrates or silicon slices which are used in the manufacture of semiconductor devices by means of deposition involving chlorosilane vapour pyrolysis.
The commercially most widespread epitaxial reactors can be divided into two main categories:
a) single-slice reactors, which are able to process a single slice at a time; and
b) batch-type reactors, which are able to process a plurality of substrates or slices at the same time.
The heating systems used for the abovementioned reactors may be classified as two types: lamp-type heating systems and medium or high frequency induction heating systems.
The batch-type reactors which are most widespread on an industrial level are essentially of two types: those which use the so-called xe2x80x9cbarrelxe2x80x9d system, i.e. with a prismatic or truncated-pyramid susceptor, and those which use the xe2x80x9cpancakexe2x80x9d system, with a substantially flat disk-shaped susceptor.
Typically, at present, batch-type reactors are of the manual loading type, whereas single-slice reactors are of the automatic loading type.
In automatic-loading reactors, the substrate, or slice, may be handled in different ways which offer both advantages and disadvantages. Handling of the substrates is particularly critical in the sector of semiconductors and, in particular, in epitaxial reactors where there are temperature-related problems and problems resulting from particle contamination.
Generally, each substrate or slice has a bottom side (back), a top side (front) and a side wall (edge). The dimensions of the front and back are normally between 75 and 300 mm, and even as much as 400 mm, while the dimensions of the edge are smaller than or near to 1 mm. The front is the most important part of a slice because it is the part where the chemical reaction process, i.e. deposition, takes place.
For the abovementioned reason, it is important to avoid all contact between the front and any type of tool used for handling, because any contact, even of the slightest nature, causes imperfections in the crystal lattice. If the imperfections are formed during loading, they are magnified by the ensuing heat process; however, imperfections introduced during unloading must also be avoided.
Basically, it may be stated that contact, even of an accidental nature, with the front of the slice is nor permitted at any time. On the other hand, within certain limits, contact with the back and with the edge of the said slice is permitted.
Therefore, in order to move a slice, it is possible to act via the front (without any direct contact, however), the back or the edge.
Basically, there is only one system which allows handling from the front, without contact between tool and slice, and it is the system based on the Bernouilli effect, whereby, by providing a suitable gripping tool (end effector), it is possible, by blowing filtered inert gas, towards the front of the slice, to create an attraction effect for the slice, which, in the horizontal position, is sufficient to overcome the weight of the slice, keeping it suspended.
However, accidental contact between the edges of the slices and some fixed points of the tool necessarily occurs because, in the absence of a support and hence friction, it is necessary to have some fixed points for fixing the slice underneath the gripping tool, although this fact is not particularly negative.
During unloading, in order to raise the slice from the cavity inside which it is seated, it is necessary to overcome, in addition to its own weight, also a slight vacuum which is formed between the cavity and the said slice. Since this is not possible by means of the Bernouilli effect alone, it is necessary to avoid the formation of this vacuum, for example by means of a network of tiny channels which are formed in the cavity underneath the slice. This technique is excellent, but is better suited for lamp-heated reactors, rather than induction-heated reactors, because the presence of non-conducting channels in the graphite mass of the susceptors would adversely affect the uniformity of heating of the slices. Moreover, this technique is not particularly compatible with reactors of the batch type because, although a flow of inert gas helps in keeping the front of the slice clean during handling, it is probably harmful for the adjacent slices, especially if handling takes place in the vicinity if the susceptor, because the flow of gas moves any dust particles which are present in movement.
Another system consists in handling the slices from the back, although there is the problem that the back of the slice is accessible when the latter is inside the cassette, but is no longer so when the slice is located on the susceptor. In order to overcome this drawback, it is possible to form through-holes in the susceptor and raise the slice, when required, by means of small supports, passing through the holes, which are able to move up to perform raising and move down to allow seating of the slice in the susceptor. In fact, during the loading cycle, the supports are raised and the slices rested on them. Then the supports are lowered and the slices are deposited in the corresponding seats on the susceptor. During the unloading cycle, the supports are raised, together with the slices; a tongue or gripping tool (end effector) made of suitable material is then introduced underneath the slice and the latter removed. If greater stability is required, it is possible to brake the slice by applying a slight vacuum between slice and gripping tool. However, this technique, although being effective, in practice can only be properly applied to reactors of the lamp-heated type, while it is probably unacceptable for induction-heated reactors because the holes formed in the graphite of the susceptor would result in a non-uniform current flow and hence heating.
Another known system is that which allows the slice to be gripped along its external diameter, or edge, at two or more points using movable gripping systems, such as mechanical grippers. However, this system cannot be easily realized, since it requires special machining of the cavity or the cavities of the susceptor which are not entirely compatible with the induction heating system. Finally, as already mentioned, any direct contact with the front of the slice is not permissible.
There exist, however, systems which are able to perform gripping of the slice from the front, limiting the contact to one or more areas on its external rim.
The slice is retained by a vacuum system by means of a chamber formed between slice and gripping tool. In this case, however, the raising force is limited to the surface area of contact with the external rim of the slice and an even slightest error in positioning between slice and gripping tool results, respectively, in a smaller or greater contact surface area with risks of lack of gripping of the slice or an increase in defects due to the direct contact between tool and slice. By way of conclusion, this system is not of an optimal nature owing to an excessively large contact surface area between tool and front of the slice.
The object of the present invention is that of providing an improved automatable device for supplying and removing semiconductor slices to/from an epitaxial reactor such as that described in the said International Patent Application WO 96/10659 which covers an epitaxial reactor provided with a flat disk-shaped susceptor and having a gas flow parallel to the substrates.
In short, operation of the reactor and the associated device comprises the following steps:
positioning racks or cassettes containing the slices to be grown inside the reactor,
loading of the product, where the slices are transferred inside the reaction chamber, as described in greater detail below;
brief hydrogen purging inside the reaction chamber;
heating so as to bring the susceptor and the slices up to the appropriate temperature;
processing cycle as required by the relevant specification;
cooling to a temperature compatible with the unloading step; and
unloading of the grown slices and transportation back into the cassettes, as described in greater detail below.
The loading and unloading steps are performed at temperatures which are compatible with the material forming the gripping tool.
Each growing cycle may be followed by other growing cycles or by a so-called etching cycle where the slices are not loaded and the following operations are performed:
brief purging by means of hydrogen in the reaction chamber;
heating so as to bring the susceptor up to the appropriate etching temperature;
etching cycle as required by the relevant specification; and
cooling to a temperature compatible with loading of slices following the etching step.
The epitaxial reactor forming the subject of the abovementioned International Patent Applciation WO 96/10659 is of the so-called xe2x80x9cpancakexe2x80x9d type, namely with a disk-shaped susceptor which is induction-heated, so that none of the systems illustrated hereinabove may be effectively used.
In order to overcome the abovementioned drawbacks, a solution is used, comprising:
a first reactor zone, towards the so-called clean room, intended to receive the cassettes containing the slices to be processed and those already processed, where this part of the reactor may be in an air atmosphere or, alternatively, may envisage a chamber for purging with an inert gas, at least at the ambient temperature for the silicon of the slice to be processed (an inert gas, even low-cost such as nitrogen, may be preferable to air). Below the more simple case of an air atmosphere is described, where neither particularly efficient gas seals nor prolonged washing with inert gases is required. In the case where air is used, only a so-called absolute filter for the air is added in order to keep the air atmosphere as free as possible from solid particles (dust). Moreover, the same first zone of the reactor is provided with two doors which can be opened at any time, so as to allow the removal of the cassettes containing the slices, where opening of the doors does not require particularly long purging or washing cycles with inert gases. However, by way of alternative, should processing of the slices make unacceptable even the smallest defects due to local oxidation of the said slice, the first zone of the reactor may be provided with sealed doors and a system for purging, also by means of a pneumatic extraction pump, and introduction of inert gas, in order to minimize exposure of the slice to the air and consequently the said defects. A first robot, called external robot controlling handling of the slices, exists, the slices being handled from the back since the present cassettes which are commercially available are constructed precisely to allow this type of handling. A gripping tool of the external robot, which is also commercially available, transports the slices, keeping them pressed in position by means of a small vacuum source which is available in the robot.
The operating principle of the abovementioned solution is explained hereinbelow.
During loading, a slice is removed from the corresponding cassette and is positioned in an alignment and centring station which may also be constructed inside the said external robot. The cassettes, the alignment and centring station and the external robot are not novel and do not form part of the present invention.
Here the slice is oriented at the desired angle and the position of its geometric centre is calculated so as to allow subsequent precise gripping. The slice is then positioned inside the purging chamber on top of a quartz disk which acts as a support and is shaped so as to allow handling by means of the commercial gripping tool of the said external robot. The quartz disk has a relatively large mass compared to the slice so as to dissipate better its heat during unloading.
The access door is closed and the purging chamber is washed with an inert gas, in order to remove completely every trace of air, and, if necessary, washing may be aided by a vacuum extraction cycle in order to accelerate the said purging operation. At this point, the door providing access to the chamber where the internal robot according to the invention operates is opened, said chamber operating always in an inert-gas atmosphere and, except during maintenance operations, never being exposed to the air.
The gripping tool, or hand, of the internal robot forms part of the present invention, which invention allows the slices to be suitably handled by means of a very limited contact with their edges. In fact, each slice has a chamfered part or edge extending over about 1 mm. The part of the gripping tool in contact with the edge of the slice is shaped so as to limit the contact zone to the sole chamfer of the edge, extending over about 1 mm, and is made of suitable material, such as quartz. This part of the slice is not, however, useful in the manufacture of integrated circuits and therefore this solution is not damaging for the quality of the finished product. Moreover, the gripping tool, or hand, is connected to the arm of the internal robot by means of a structural pipe which has two functions:
the first is that of extending the robot arm so as to reach the position of the susceptor inside the reaction chamber;
the second is that of achieving, by means of an articulation or by exploiting the flexibility the arm, a certain degree of self-levelling between tool, or hand, and slice which is used during the operations involving raising of the said slice.
Raising of the slice is ensured by a certain vacuum which is obtained by a dedicated pneumatic machine. The vacuum is transferred to the slice by means of a series of holes which are distributed along the periphery of the gripping tool and are concentrated in the round zone of the slice and absent in a flat zone or recessed notch zone which is used for identification and orientation of each slice, where the flat zone is preferred for slices with a diameter of up to 150 mm, whereas in the case of larger-diameter slices the recessed notch zone is preferred. Since every irregularity in the shape of the slice causes a local reduction in the effects of the vacuum, according to the invention it is necessary to offset this reduction with a suitably calculated concentration of suction holes in the gripping tool.
During loading, the susceptor rotates and correctly positions, by means of reference systems known per se, the cavity to be loaded. This positioning may be performed by means of optical systems which are known per se. Then the slice is introduced into the reaction chamber and positioned above a suitable cavity of the susceptor. The internal robot moves downwards slightly, brings the slice into contact with the cavity and, when the vacuum is removed, releases the slice which is loaded onto the said cavity. The loading steps are repeated until all the cavities present on the susceptor are occupied. The scheduled processing of the slices is then started.
Out of all the optical systems it is possible to use a laser telemeter system which measures the distance between a laser emitter and the susceptor in question, producing an analog signal which is proportional to the said distance. For example, the laser telemeter helps eliminate defects in the parallel positioning of the susceptor by means of a scanning algorithm and subsequent correction by mechanical means which are known per se (for example micrometer screws). This parallel positioning is essential for ensuring uniformity of the epitaxial depositions onto the slices.
Once the scheduled processing of the slices has been completed, it is necessary to unload them from the susceptor. To do this, after waiting for the susceptor to reach a suitable temperature allowing extraction, without damage, of the slices from the reaction chamber, it is necessary to use again the said internal robot, which removes each slice from the corresponding cavity of the susceptor and transports it inside the purging chamber where it is placed onto the quartz disk which, with its large mass, cools it. After sufficient cooling, the slice is transferred by the external robot to a seat of one of the cassettes located in the first zone of the reactor.
The present invention which implements the methods described above consists in a device for handling substrates of materials produced in epitaxial apparatuses or reactors, such as slices of semiconductor materials, comprising:
a reaction chamber,
an internal robot for handling the substrates or slices of semiconductor materials,
a purging chamber for passing the slices through a cleaning atmosphere,
a storage zone containing cassettes which house, in a stacked arrangement, the slices of semiconductor materials,
an external robot for transferring the slices from the store to the purging chamber,
the internal robot comprising a sealed chamber which houses an articulated arm having a gripping means at its external end,
characterized in that the gripping means of the internal robot comprises at least one arm which can be inserted into the reaction chamber and terminating in a gripping tool or hand for removing a slice of semiconductor material from the purging chamber and transporting it, after passing through the sealed chamber, so as to be deposited in a recessed seat of a disk-shaped susceptor of the reaction chamber, and vice versa, from the recessed seat to the purging chamber, where the hand is designed to contact the slice on its uppermost surface along a peripheral zone or chamfered edge and the hand is adapted to grip the slice by means of a vacuum effect, and where the gripping means includes means for attaching it pivotally to the articulated arm in such a manner that when depositing the substrate in the recessed seat the substrate is held inclined to the plane of the recessed seat so as to touch the recessed seat initially only with the slice edge located farthest from the arm of the gripping means before then pivoting about said edge touching the seat to become fully in contact therewith, and vice versa, when transporting a slice from the reaction chamber to the purging chamber, the slice gripped by the hand""s vacuum effect is detached from the recessed seat by first lifting the side of the slice nearest the arm of the gripping means and then detaching the substrate completely, and raising the substrate further whilst being held inclined to the plane of the recessed seat.
In particular the arm is tubular and hollow, being connected, on one side, by means of a flexible pipe to a vacuum source and, on the other side, to a circular seat formed inside the hand so as to apply a vacuum between a bottom side of the said hand and a slice present underneath the hand.
Preferably, articulation means are arranged between the hollow tubular arm and the articulated arm of the internal robot, said means allowing the hollow tubular arm to be raised and lowered so as to bring the hand above and below a plane defined by the articulated arm.
In addition, articulation means are arranged between the hollow tubular arm and the articulated arm of the internal robot, said means also allowing a rotation of the hollow tubular arm about its longitudinal axis.
Preferably, the articulation means comprise a support bearing which is fixed to the articulated arm and carries a rotating pin about which the articulation means rotate, and an adjusting screw for fixing the permitted heights for raising and lowering of the hand with respect to the plane of the articulated arm, so that the hand can rest in the radial direction on the disk-shaped susceptor only with the front part, only with the rear part or perfectly level with the said susceptor.
In addition, the articulation means also comprise, around the hollow tubular arm, anti-friction bushes for allowing alignment of the said hand, in a direction perpendicular to the radius of the disk-shaped susceptor.
Even more preferably, the hand is in the form of a disk with a diameter greater than the diameter of the slice to be handled and has a bottom part, facing the slice, provided with a recessed seat which engages only with an external peripheral rim of the said slice.
Preferably, the seat is present on a bottom part of the hand and is provided with a plurality of peripheral holes connected to a chamber inside the said hand which, in turn, is connected to the hollow tubular arm so as to apply a vacuum between the slice and the hand.
Most preferably, the peripheral holes are concentrated mainly where the greatest losses in vacuum are envisaged.
In a particular embodiment, a laser telemeter is used to measure a distance between a laser emitter and the disk-shaped susceptor of the reaction chamber, in question, producing an analogue signal proportional to the said distance, where said telemeter detects defects in the level arrangement of the susceptor as well as defects in the parallel positioning of the susceptor with respect to the reaction chamber.
Furthermore, a notch formed in the external rim of the susceptor is used as an angular reference point which can be detected by the laser telemeter and the recessed seats for the slices are counted starting from this notch.
According to the present invention, a method for placing a slice in a recessed seat of a disk-shaped susceptor, present in a reaction chamber, by means of a hand of a device, as defined above, is also provided, characterized in that a slice, which is made to adhere to the hand by means of a vacuum, enters into the reaction chamber in the raised position, is transported to above one of the recessed seats of the disk-shaped susceptor, is lowered so as to be placed onto the said seat, remaining inclined forwards so as to touch the recessed seat initially only with the slice edge located farthest from the arm of the gripping means and then with its whole surface, and then, after removal of the vacuum which keeps the slice attached to the hand, is further lowered, detaching the hand from the slice, and then the hand is raised again, being detached entirely from the slice, and finally is retracted, leaving the reaction chamber.
Alternatively, the method for removing a slice from a recessed seat of a disk-shaped susceptor, present in a reaction chamber, by means of a hand of a device, as defined above, is characterized in that the hand enters into the reaction chamber in the raised position, is transported to above a slice housed in one of the recessed seats of the disk-shaped susceptor, is lowered until it touches the slice, first at the edge located farthest from the arm of the gripping means and then over the entire circumference of the slice and then, after touching the slice over the entire circumference, applies a vacuum so as to cause the slice to adhere with its edge onto the hand and then starts to move up again, detaching the slice from the recessed seat, first at the edge located nearest the arm of the gripping means and then over the entire surface of the slice, and finally, after the slice has been raised completely from the recessed seat of the susceptor, transports it outside the reaction chamber.